1. Field of the Invention
This invention relates to integrated circuits in general and, more particularly, to metal conductors on integrated circuits.
2. Description of the Prior Art
Very large scale integrated circuits, such as microprocessors, have logic circuitry (buffers) which drive external logic circuits, such as buffers for driving an address or data bus. Compared to the logic circuit which are internal to the microprocessor, the buffers supply relatively large output currents to drive the external logic circuits. Hence, the transistors in the buffers are designed to handle more current than their counterparts in the internal logic circuitry.
A consequence of using high currents in an integrated circuit (IC) with metal conductors is electro-migration of the metal. Aluminum conductors (or aluminum alloyed with copper) suffer from electro-migration when the current density in the conductor exceeds a predetermined maximum amount. Electro-migration can lead to premature failure of the IC unless proper care is taken to ensure that the current density in any one conductor does not exceed the predetermined maximum. To keep the current density below the predetermined maximum, the metal conductors, within and coupling from the buffers, are therefore larger (wider) than the metal conductors interconnecting the internal logic circuitry.
For the buffer to achieve the larger current carrying capacity, a common approach utilizes multiple parallel transistors therein, each transistor being similar in size to other transistors in the IC. Each transistor, via relatively thin metal conductors, couples to a relatively wide metal conductor designed to carry the larger current. This approach suffers from wasting large amounts of area on the IC for the buffers due to the relative area inefficiency of the wide metal conductors coupling to the parallel transistors.
Still another common approach utilizes one or more relatively large transistors in parallel with the relatively wide metal conductor coupling directly to each transistor. The width of the metal conductor is chosen to minimize the electro-migration of the conductor under a maximum design current flow. As with the multiple transistor approach, above, this approach suffers from wasting large amounts of area on the IC for the buffers due to the inefficient use of the wide metal conductors to couple to the relatively large transistors.
It is noted that the cost of having larger than necessary buffers, of which there can be hundreds in a particular IC, is the reduced area available on the IC for other logic circuits. This constraint can detrimentally impact the functions or features desired from the IC.